The perfecting of imaging processes has caused important progresses in medicine. Nuclear magnetic resonance imaging (MRI) is desirable alternative to invasive angiography. It allows making visible tissues and blood vessels. For example, cardiac MRI has been shown to provide accurate image of the proximal and medial parts of the coronary arteries. Since the presence of contrast agents or anesthetics is not necessary for MRI-based angiography, there is minimal risk of complications with MRI. One of the advantages of MRI is that the blood flow can be assessed quantitatively. This flow information can provide important data concerning i.a. the presence of stenosis in the vessel. However, MRI does have some limitations in the presence of conventional metallic medical devices. For instance, medical devices such as vascular prosthesis and stents often comprise, or essentially consist of, a metallic substrate in order to obtain adequate mechanical properties, and the ferro- or ferri-magnetic (FFM) properties of the metallic substrate cause the production of artifacts. An artifact is a feature appearing in an image that is not present in the original object and it can misrepresent the anatomy under diagnosis using MRI by either partially or completely blocking out the desired image in the vicinity of the metal parts.
For example, after a stent has been introduced in a vessel of a patient, it is generally advisable to continuously monitor its efficiency in order to detect any undesirable in-stent restenosis. However, since stents often comprise metal parts (wire or plates), excessive signal loss is observed inside and close to the stent.
US 2005/0276718A discloses a biocompatible alloy used for implantable medical device, wherein the MRI compatibility of the alloy is improved by reducing the iron and/or silicon contents thereof. Namely, the MRI compatibility of an alloy has been improved by optimizing the composition ratio of the alloy per se.
EP 1864149 discloses a stent exhibiting reduced interference in MRI. The stent consists of NiTi alloy such as Nitinol containing at least 50 weight percent of nickel (wt % Ni). The exterior surface of stent is modified so as to be covered with an oxide layer wherein all nickel atoms present in the oxide layer are oxidized into nickel monoxide (NiO). This document also discloses a method for providing a nickel monoxide layer at the exterior surface of Nitinol. This method is only applicable to the nickel-based alloy comprising at least 50 wt % Ni.
Ferromagnetism is the basic mechanism by which certain materials form permanent magnets, or are attached to magnets. In physic, several different types of magnetism are distinguished. Ferromagnetism including ferrimagnetism is the strangest type; it is the only type that creates forces strong enough to be felt, and is responsible for the common phenomena of magnetism encountered everyday life. For example, cobalt, iron and nickel are known as ferromagnetic material in its metallic state. Some metals such as iron and chromium are known as ferri- or ferro-magnetic (FFM) material in its oxidized state.
A significant portion of stents used in a clinical setting are made of cobalt-based alloy, such as Phynox, Elgiloy and Cobalt-Chromium. Usually a cobalt-based alloy has high atom concentration of cobalt and often chromium which are ferromagnetic materials in its metallic state or its oxidized state, and produce strong artifacts in MRI. If an alloy does not have a sufficient concentration of nickel comparing to the FFM material(s) present in the alloy, the conventional method of providing nickel monoxide layer as an anti-artifact layer is not applicable. Furthermore, if an alloy comprises a metal which exhibits the FFM property in its oxidized state, e.g., chromium oxide and iron oxide, simply converting all metal present at the outermost surface of the alloy into oxidized state in order to obtain an oxide layer including NiO is not always sufficient to obtain a sealing (masking) effect of the FFM property of the alloy for MRI visibility because it may increases the surface concentration of FFM material in its oxidized state.
Therefore, a new composition used as anti-artifact layer deposited at the outermost surface of a metallic substrate, particularly an alloy comprising (a) less than 50 wt % Ni or without Ni and (b) metal(s) which exhibits the FFM property in its oxidized state, is desired. Furthermore, a method for providing an anti-artifact layer at the outermost surface of said metallic substrate is longed for. The object of the present invention is to provide a solution for satisfying the requirements mentioned above.